1. Field of the Invention
This invention relates in general to a thermal asperity detector, and more particularly to a thermal asperity detector that is able to detect short thermal asperities and which uses shorter time thresholds.
2. Description of Related Art
In data channel for a magnetoresistive (MR) sensor, a transient disturbance can result from a "thermal asperity". When a hard particle trapped on the surface of a magnetic disk collides with a MR sensor riding closely adjacent to the disk surface, a rapid temperature rise occurs in the sensor. This friction-created temperature increase of up to 120.degree. C. first occurs at the point of contact between particle and MR sensor. The localized temperature increase produces a small but sudden increase in temperature of the entire MR sensor, perhaps as much as several centigrade degrees averaged over the whole sensor, within 50 to 100 nanoseconds. Because the MR sensor has a non-zero temperature coefficient of resistance (e.g. 0.003/.degree. C. for permalloy), the sensor resistance then increases in response to the sudden temperature rise.
The heat conducted into the MR sensor from the localized hot spot diffuses slowly from the sensor to the local environment, causing the typical delayed exponential decay known for such thermal asperities. For instance, the increased sensor resistance can be seen to decline about 30% within the first one-half to five microseconds following collision.
Because the MR sensor detects magnetic signals by exploiting the magnetoresistive effect, resistance changes arising from magnetic changes on the disk surface adjacent to the sensor are detected as changes in voltage across the sensor. A DC bias current induces the voltage across the sensor resistance that varies according to changes in the sensor resistance. Thus, a thermal a sperity induces a superimposed voltage transient on the desired data signal from the sensor. Because MR sensor non-linearity increases with increasing magnetic signal excursions about the sensor bias point, the sensor is designed to keep the magnetic excursions induced by desired data signals sufficiently small to ensure reasonable sensor linearity. For instance, detection of a magnetic change on the disk surface may require only a 0.3 percent change in sensor resistance. Thus, thermal asperity transients can exceed 400 percent of the typical base-to-peak magnetic data signal voltage amplitude from the MR sensor.
Thermal Asperity (TA) detectors are used to detect anomalies in a disk read signal that are caused by heating of the head's magnetoresistive sensor as it strikes a disk asperity. Previous thermal asperity (TA) detectors work well with large (in amplitude and duration) thermal asperities, but do not detect smaller asperities well. Previous detectors also have difficulty accurately detecting asperities when the signal rings after the overshoot caused by the low-frequency coupling pole.
One solution that has been used is described in IBM Technical Disclosure Bullet, entitled "Digital Thermal Asperity Detection", Vol. 34, No. 6, November 1991, pp. 338-9, hereby incorporated by reference, involves counting the number of samples that an asperity exceeds a threshold ("saturates"). If the count exceeds a given value, an asperity is declared. This method works well with large duration asperities, but not small, since the signal often decays too fast for more than one sample to saturate.
Another method that has been used is to declare a thermal asperity when the signal samples are either above a given threshold or below a given threshold for a given number of clock periods. This method has two disadvantages. First, the detection as implemented only detects in one direction (that is, it only detects either positive or negative thermal asperities). This precludes the detection of "cooling thermal asperities" that are of the opposite polarity than the ordinary "heating thermal asperities." Secondly, the method doesn't work for smaller thermal asperities because the threshold must be set below the partial-response maximum-likelihood (PRML) channels O's level, and DC erases (or long magnets) are detected as thermal asperities if the time threshold is set too low.
The increasing number of disk drive manufacturers that are using magnetoresistive heads in their drives need to detect thermal asperities during the manufacturing process to ensure that these disk defects are screened out. Detection of thermal asperities during a product's life can allow the drive to remap bad sectors before the loss of customer data.
It can be seen then that there is a need for a thermal asperity detector that tests a signal to see if the signal saturates for one sample before applying a level/time threshold to confirm the thermal asperity.
It can also be seen that there is a need for a thermal asperity detector that allows short thermal asperities to be detected by applying level/time thresholds after a potential asperity is seen.
It can also be seen that there is a need for a thermal asperity detector that allows for the use of shorter time thresholds.
It can also be seen that there is a need for a thermal asperity detector that saves power over prior methods by delaying the enablement of the level/time threshold circuit until after a saturated asperity has been detected.